The specifications for mobile phones, except for the NMT450 network in the Nordic countries, require the use of a compander. The compander is used to improve speech quality. The companding reduces noise and different interferences originating in the radio path.
The compander is employed in the audio part of the mobile phone. Present solutions use a compander realized in prior art bipolar technology. A prior art compander comprises two sections, the compressor and the expander. The compressor compresses with a ratio of 1:2 the dynamics of the signal passing through it, and the expander accordingly expands with the ratio of 2:1 the dynamics of the signal passing through it.
FIG. 1 shows the operating principle of a bipolar compander according to prior art. In its IC implementation the compressor 1 and the expander 2 are two substantially identical blocks within the same IC circuit. The basic component blocks are a full wave rectifier 3 and 4, an adjustable amplifier cell 5 and 6, an operational amplifier 7 and 8, and the internal biasing of the circuit. With external components it is possible to realize the functions of either compression or expansion. This takes place by connecting the adjustable amplifier cell 5, 6 and the full wave rectifier 3, 4 either to the feedback chain (compressor) or the input of the amplifier (expander).
FIG. 2 shows the operating principle of a compander, which differs from the generally known compander embodiment and forms the basis when the compander according to the invention is realized.
The compressor section 9 comprises identical amplifier sections 11 and 12 with gain G. The input signal Uin is connected through both amplifier sections 11, 12 to the full wave rectifier block 13 having the input signal level Uref=G*G*Uin. The amplifiers are controlled according to this information so that the desired compression is achieved. The output signal Uout=G*Uin is obtained at the output of the first amplifier 11.
The first amplifier 14 of the expander section 10 is an inverse amplifier of the respective G-amplifier and its gain is 1/G. The input signal Vin is also connected directly to the block 15 operating with gain G. From this block 15 the signal is connected to the full wave rectifier block 16, the rectifier 16 having an output level Uref=G*Uin. According to this information the amplifiers 14 and 15 are controlled so that the desired expansion is obtained. The output signal Uout=1/G*Uin is obtained at the output of the first 1/G-amplifier 14.
The amplifier gain changes until the signal Uref supplied to the full wave rectifier block 13, 16, after a certain response time, reaches a level of constant dynamics. This level is called the ineffective level, the reference level, at which a signal passes through the compressor 9 and the expander 10 with unaltered dynamics. The operating ranges of the compressor and the expander are formed relating to this reference level.
The widely known operating principle shown in FIG. 2 does not stipulate in which way the amplifier control should be realized. It is worth noting that full wave rectification is used to detect the level of the signal to be companded. The amplifier gain is changed according to this level information, so that the desired compression or expansion is obtained. The prior art compander realized in bipolar technology also uses full wave rectification to control the amplification.
U.S. Pat. No. 4,987,383 discloses an integrated compression amplifier that can be constructed in CMOS technology and whose threshold voltages are programmable. The amplifier utilizes switched capacitor amplifiers.
A basic problem is that the prior art compander embodiment poorly suites present mobile phone applications. Due to its encapsulation it requires a large area on the circuit board. It further requires several external components. The main problem of the basic solution described in FIG. 2 is on the other hand how to realize it in practice.
If the compander is realized in CMOS-technology and full wave rectification of the signal is used, there is an alternative in the use of A/D-converters, so that it will be possible to control the controlled amplifiers with which the companding of the payload signal can be realized accurately. The circuitry for the A/D-conversion would be complicated. On the other hand, if a full wave rectified analog signal is used to control the amplifiers, the amplifier realization will be complicated in order to obtain the required linearity and dynamics.
The mobile phone specifications define so-called transient response times for the compressor and the expander, an attack response time tA determined by the sudden increase of the signal, and a return time tR due to the sudden decrease of the signal. These times determine how fast the output of the compressor or the expander will follow changes in the input level. The compressor has a tA of about 3,5 ms and a tR of about 13,5 ms. The expander tA and tR are about 13,5 ms. Realization of these differing transient response times is problematic.
When CMOS-technology is used, and when the smallest possible silicon area is sought after, the amplifiers must be realized as so-called switched capacitor amplifiers, i.e. or SC-amplifiers (Switched Capacitor), which are based on the use of clock controlled capacitors. Then there is a problem to get a correct timing of the sampling of the processed payload signal. The offset voltages and the coupling of different interferences to the payload signal should be minimized.
The mobile phone specifications demand that it must be possible to switch off the compression and expansion functions in some situations, for example in the type approval measurements of the mobile phones. When it is desirable to use the same compander in different mobile phone systems, in AMPS, NMT and in TACS, differing audio line levels in the mobile phone transmission and reception paths will pose a problem. Then it will be inconvenient to switch off the companding or to implement the by-pass mode so that it would be suitable for all systems.